Jack Straw today became the first British minister to raise questions about the handling of the release of the Lockerbie bomber, Abdelbaset al-Megrahi.

As opposition politicians criticised Gordon Brown for failing to express an opinion on Megrahi's release, the justice secretary indicated that he would have taken a different approach to that of his Scottish counterpart, Kenny MacAskill.

Speaking at the Guardian's offices, where he was attending a seminar on House of Lords reform, Straw reiterated Brown's position that it would be wrong for a UK minister to offer an opinion on a decision taken in Edinburgh.

But he questioned whether it was right for MacAskill to visit Megrahi in prison as he decided whether to release him.

Megrahi, who has terminal prostate cancer, was released on compassionate grounds last Thursday.

Asked whether it was right for MacAskill to visit him in Greenock prison, Straw said: "That was his decision.

"If you are asking me if I have ever visited a prisoner in jail who has applied for compassionate release, the answer to that is no."

Straw has considered requests for release, on compassionate grounds, from prisoners in England and Wales over six and a half years, first during his time as the home secretary between 1997 and 2001 and then as the justice secretary since 2007.

MacAskill said he was obliged to visit Megrahi in prison as he considered whether to transfer him to Libya under a prison transfer agreement negotiated by the British and Libyan governments.

The Scottish justice secretary released him on compassionate grounds, using existing Scottish law, after saying the British government had failed to answer questions he had about the prisoner agreement.

Straw said he had answered questions about the agreement from Scotland, pointing out that he had told the Scottish first minister, Alex Salmond, that prisoners could make representations in writing.

"I said in a letter to Alex Salmond that the prisoner would have a right to make written representations which, for sure, is true," Straw said.

His advice to Salmond is being used by the Scottish Labour party to criticise the release of Megrahi.

Straw declined to offer an opinion on the merits of the decision on two grounds, saying he had not seen the evidence and it would be wrong for a UK minister to interfere in a Scottish decision.

"I cannot offer you a view about that decision because I have not seen the evidence," he said. I know from long experience you have to look at the evidence.

"As the prime minister said yesterday, this is a decision for the Scottish executive.

"Look, it is for Scots to decide their own processes. Their legal system has been different for ever. It has not just been different since 1999. It has been different since the Act of Union in 1707."

The justice secretary denied that it had been convenient for London – which wants to improve business links with Libya while maintaining warm relations with the US – that such a sensitive decision had been taken in Edinburgh.

"I don't think it is an issue of convenience," he said. "If it had not been taken by the Scottish executive, it would have fallen to the Scottish secretary of state. It is not an issue of convenience."

Straw, who faced criticism over his decision to release the train robber Ronnie Biggs on compassionate grounds, offered some sympathy for MacAskill, who has been condemned in the US for giving succour to terrorists.

"Even if you have seen the evidence, there is a world of difference between having to come to a decision, which is a binary choice – do you let this individual out or not? – and just making observations from the sidelines," he said.

Straw's former ministerial colleague John Prescott said he did "not have any objection" to the decision to release Megrahi, and rallied to Brown's defence over his refusal to express an opinion on the matter.

"It's clear that there are other issues ... which the PM has been asked about, and he answered yesterday that governments always have difficult decisions to take," the former deputy prime minister told Sky News.

"This was clearly one, without getting into the arguments of whether this man was guilty or innocent. The courts found him guilty, it's on compassion grounds, under the legal system he is now being released.